Infrared Spectra of Simple Methylidyne, Methylidene, and Insertion

Organometallics 2010, 29, 2211–2222 DOI: 10.1021/om900902p

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Infrared Spectra of Simple Methylidyne, Methylidene, and Insertion Complexes Generated in Reactions of Laser-Ablated Rhodium Atoms with Halomethanes and Ethane Han-Gook Cho and Lester Andrews* Department of Chemistry, University of Incheon, 177 Dohwa-dong, Nam-ku, Incheon, 402-749, South Korea, and Department of Chemistry, University of Virginia, P.O. Box 400319, Charlottesville, Virginia 22904-4319 Received October 14, 2009

Small rhodium carbene complexes, CX2dRhX2, CXHdRhX2, and CH2dRhX2, are identified in the product matrix infrared spectra from reactions of laser-ablated Rh atoms with tetra-, tri-, and dihalomethanes. Evidence for Rh carbynes, XCtRhX3, is also found in the tetrahalomethane spectra. Calculated Rh-C bond lengths of 1.788-1.820 and 1.730-1.740 A˚ are appropriate for double and triple bonds, respectively. While only the CH3-RhF insertion primary product is observed in the CH3F spectra, weak RhH and RhD diatomic molecule absorptions suggest that the C-H activation product CH2F-RhH, containing a Rh-H bond, also forms during reaction, but subsequently dissociates. The analogous CH3CH2-RhH insertion product is identified in the ethane spectra, in line with the recently discovered CH3-RhH. The observation of RhH and RhD without dihydride or methylidene absorptions in the product spectra suggests that only the insertion complex is generated in the Rh þ C2H6 reactions. The present results reconfirm the trend that higher oxidation-state complexes become less favored moving away from W, Re, and Os in the periodic table.

Introduction Transition-metal high-oxidation-state complexes,1 since their first discovery in the 1970s,2 have been considered an essential part of coordination chemistry due to their distinct chemistry and industrial importance including catalytic properties for metathesis reactions and extensive applications in synthetic chemistry.3 Recently small transition-metal complexes with carbon-metal multiple bonds have been provided from direct reactions of group 3-8 and 10 transition metals and actinide metal atoms with small alkanes and halomethanes, through C-H(X) insertion and subsequent

H(X) migration.4-9 These new metal complexes are more amenable for higher level theoretical analysis10 and, therefore, provide new model systems to investigate coordination chemistry. The higher-oxidation-state complexes are expected to be less favored on moving far right in the periodic table due to the more filled d-orbitals, whereas halogen substitution fosters the formation of small higher-oxidationstate complexes owing to the strong metal-halogen bond and rich electron density.3,7,8 Recent studies show that tetrahalomethanes are necessary precursors to form the Ni and Pd methylidenes,8a,b while Pt methylidenes are easier to produce.8c,d Recently Zhou, et al. reported the formation of the C-H insertion product through observation of the emerging Rh-H stretching and CH3 deformation bands in

*To whom correspondence should be addressed. E-mail: lsa@ virginia.edu. (1) (a) Fischer, E. O.; Kreis, G.; Kreiter, C. G.; M€ uller, J.; Huttner, G.; Lorenz, H. Angew. Chem., Int. Ed. Engl. 1973, 12, 564. (b) Schrock, R. R. J. Am. Chem. Soc. 1974, 96, 6796. (2) (a) Herndon, J. W. Coord. Chem. Rev. 2009, 253, 1517. (b) Herndon, J. W. Coord. Chem. Rev. 2009, 253, 86. (c) Herndon, J. W. Coord. Chem. Rev. 2007, 251, 1158. (d) Herndon, J. W. Coord. Chem. Rev. 2006, 250, 1889. (e) Herndon, J. W. Coord. Chem. Rev. 2005, 249, 999. (f) Herndon, J. W. Coord. Chem. Rev. 2004, 248, 3, and earlier review articles in this series. (3) (a) Crabtree, R. H. Chem. Rev. 1995, 95, 987, and references therein. (b) Wada, K.; Craig, B.; Pamplin, C. B.; Legzdins, P.; Patrick, B. O.; Tsyba, I.; Bau, R. J. Am. Chem. Soc. 2003, 125, 7035. (c) Ujaque, G.; Cooper, A. C.; Maseras, F.; Eisenstein, O.; Caulton, K. G. J. Am. Chem. Soc. 1998, 120, 361. (4) (a) Andrews, L.; Cho, H.-G. Organometallics 2006, 25, 4040, and references therein (Review article, groups 4-6). (b) Lyon, J. T.; Cho, H.-G.; Andrews, L. Organometallics 2007, 26, 2519 (Ti, Zr, Hf þ CHX3, CX4). (c) Lyon, J. T.; Cho, H.-G.; Andrews, L. Organometallics 2007, 26, 6373 (Cr, Mo, W þ CHX3, CX4). (5) (a) Cho, H.-G.; Andrews, L. J. Phys. Chem. A 2007, 111, 2480. (b) Cho, H.-G.; Andrews, L. Organometallics 2007, 26, 633 (group 3).

(6) (a) Cho, H.-G.; Andrews, L. Organometallics 2007, 26, 4096. (b) Cho, H.-G.; Andrews, L. Inorg. Chem. 2008, 47, 1653. (Re). (7) (a) Cho, H.-G.; Lyon, J. T.; Andrews, L. Organometallics 2008, 27, 5241. (b) Cho, H.-G.; Andrews, L. Eur. J. Inorg. Chem. 2008, 2537. (c) Cho, H.-G.; Andrews, L. Organometallics 2008, 27, 1786 (group 8). (8) (a) Cho, H.-G.; Andrews, L.; Vlaisavljevich, B.; Gagliardi, L. Organometallics 2009, 28, 5623 (Ni þ CX4). (b) Cho, H.-G.; Andrews, L.; Vlaisavljevich, B.; Gagliardi, L. Organometallics 2009, 28, 6871. (c) Cho, H.-G.; Andrews, L. J. Am. Chem. Soc. 2008, 130, 15836. (d) Cho, H.-G.; Andrews, L. Organometallics 2009, 28, 1358. (e) Cho, H.-G.; Andrews, L. J. Phys. Chem. A 2008, 112, 12293 (group 10). (9) (a) Andrews, L.; Cho, H.-G. J. Phys. Chem. A 2005, 109, 6796. (b) Cho, H.-G.; Lyon, J. T.; Andrews, L. J. Phys. Chem. A 2008, 112, 6902. (c) Lyon, J. T.; Cho, H.-G.; Andrews, L. Eur. J. Inorg. Chem. 2008, 1047 (actinides). (10) (a) von Frantzius, G.; Streubel, R.; Brandhorst, K.; Grunenberg, J. Organometallics 2006, 25, 118, and references therein. (b) Berkaine, N.; Reinhardt, P.; Alikhani, M. E. Chem. Phys. 2008, 343, 241. (c) Chung, G.; Gordon, M. S. Organometallics 2003, 22, 42.

r 2010 American Chemical Society

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Organometallics, Vol. 29, No. 10, 2010

Cho and Andrews

the process of annealing after co-deposition of Rh and CH4 in solid argon, and in our laboratory these bands were observed on sample co-deposition.11 More recently CH3PtH is reported along with evidence for rearrangement of a small fraction to CH2dPtH2.8e On the other hand, Os exclusively forms carbynes in reactions with small alkanes and halomethanes, while Ru forms both carbynes and carbenes, and Fe produces methylidene and insertion complexes.7 In this study, we report matrix infrared spectra from Rh reactions with eight halomethanes and ethane. Small methylidene, methylidyne, and insertion complexes are identified in the matrix spectra, confirming the general trend that small higher-oxidation-state complexes are most favored with W, Re, and Os and become gradually less favored on moving away from them in the periodic table.4-8

Experimental and Computational Methods Laser-ablated rhodium atoms were reacted with CCl4 (Fisher), CCl4 (90% enriched, MSD Isotopes), CFCl3, CF2Cl2, CH2FCl, CH2F2 (Dupont), CHCl3, CH2Cl2 (Fisher), CH3F (Matheson), CDCl3, CD2Cl2 (MSD Isotopes), CD2FCl, CD2F2, and CD3F (synthesized12) in excess argon during condensation at 10 K using a closed-cycle refrigerator (Air Products Displex). These methods have been described in detail in previous publications.3,13 Reagent gas mixtures ranged from 0.2 to 1.0% in argon. The Nd:YAG laser fundamental (1064 nm, 10 Hz repetition rate, 10 ns pulse width) was focused on a rotating metal target (Rh, 99.99%, Johnson-Matthey) using 5-10 mJ/pulse. After initial reaction, infrared spectra were recorded at 0.5 cm-1 resolution using a Nicolet 550 spectrometer with a Hg-Cd-Te range B detector. Then samples were irradiated for 20 min periods by a mercury arc street lamp (175 W) with the globe removed using a combination of optical filters and annealed to allow further reagent diffusion. To provide support for the assignment of new experimental frequencies and to correlate with related works,4-9 density functional theory (DFT) calculations were performed using the Gaussian 03 program system,14 the B3LYP density functional,15 the 6-311þþG(3df,3pd) basis sets for C, F, and Cl,16 and the SDD pseudopotential and basis set17 for Rh to provide vibrational frequencies for the reaction products. Geometries were 13

Figure 1. Infrared spectra in the 1200-1060, 1060-760, and 550-400 cm-1 regions for the reaction products of the laserablated rhodium atom with CCl4 isotopomers in excess argon at 10 K. (a) Rh and CCl4 (0.5% in argon) co-deposited for 1 h; (b) as (a) after visible (λ > 420 nm) irradiation; (c) as (b) after ultraviolet (240-380 nm) irradiation; (d) as (c) after full arc (λ > 220 nm) irradiation; (e) as (d) after annealing to 28 K; (f) Rh and 13CCl4 reagent (0.5% in argon) co-deposited for 1 h, (g-j) as (f) spectra taken following the same photolysis and annealing sequence. m and y designate the product absorption groups, while P and c stand for the precursor and common precursor product absorptions. Cl2CCl-Cl is produced in CCl4 reactions due to the ablation plume irradiation. fully relaxed during optimization, and the optimized geometry was confirmed by vibrational analysis. The BPW9118 functional was also employed to complement the B3LYP results. The vibrational frequencies were calculated analytically, and zeropoint energy is included in the calculation of binding and reaction energies. Previous investigations have shown that DFT calculated harmonic frequencies are usually slightly higher than observed frequencies,4-9,19,20 and they provide useful predictions for infrared spectra of new molecules.

Results and Discussion (11) (a) Wang, G.; Chen, M.; Zhou, M. Chem. Phys. Lett. 2005, 412, 46. (b) Wang, X.; Andrews, L. Unpublished results, 2009 (Rh þ CH4). (12) Isotopic modifications synthesized: Andrews, L.; Willner, H.; Prochaska, F. T. J. Fluorine Chem. 1979, 13, 273. (13) (a) Andrews, L.; Citra, A. Chem. Rev. 2002, 102, 885, and references therein. (b) Andrews, L. Chem. Soc. Rev. 2004, 33, 123, and references therein. (14) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A. Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision C.02; Gaussian, Inc.: Wallingford, CT, 2004. (15) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang, Y.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (16) Raghavachari, K.; Trucks, G. W. J. Chem. Phys. 1989, 91, 1062. (17) Andrae, D.; Haeussermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Theor. Chim. Acta 1990, 77, 123.

Reactions of rhodium atoms with halomethanes and ethane were investigated, and the infrared spectra (Figures 1-8) and density functional frequency calculations (Tables 1-9) of the products and their structures (Figure 9) will be presented in turn. Rh þ CCl4. The reaction product spectra are shown in Figure 1 from laser-ablated Rh atoms co-deposited with carbon tetrachloride in excess argon during condensation at 10 K. Two strong, new absorptions at 964.3 cm-1 (with a satellite at 958.6 cm-1 for matrix site splitting) and 867.2 cm-1 (with a shoulder at 865.4 cm-1 for chlorine isotopic splitting) (labeled m for methylidene) are observed in the C-Cl stretching region.21 Another strong m absorption is observed at 410.7 cm-1 (with 407.4 cm-1 shoulder for chlorine isotope splitting). Absorptions produced previously (18) (a) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (b) Burke, K.; Perdew, J. P.; Wang, Y. In Electronic Density Functional Theory: Recent Progress and New Directions; Dobson, J. F., Vignale, G., Das, M. P., Eds.; Plenum: New York, 1998. (19) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502. (20) Andersson, M. P.; Uvdal, P. L. J. Phys. Chem. A 2005, 109, 3937. (21) Pavia, D. L.; Lampman, G. M.; George, S. K. Introduction to Spectroscopy, 3rd ed.; Brooks Cole: New York, 2000.

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Figure 2. Infrared spectra in the 1600-1400, 1400-900, and 550-400 cm-1 regions for the reaction products of the laserablated rhodium atom with CF2Cl2 and CFCl3 in excess argon at 10 K. (a) Rh and CFCl3 (0.5% in argon) co-deposited for 1 h; (b-e) as (a) spectra taken following the irradiation and annealing sequence described in Figure 1 caption (visible, UV, and full arc irradiations and annealing to 28 K); (f) Rh and CF2Cl2 reagent (0.5% in argon) co-deposited for 1 h, (g-j) as (f) spectra taken following the same irradiation and annealing sequence. m and y designate the product absorption groups, while P and c stand for the precursor and common absorptions.

through vacuum ultraviolet irradiation at 1036.4 cm-1 (CCl3þ),22 1019.3, 926.7, 501.9 cm-1 (Cl2CCl-Cl),23 and 898 cm-1 (CCl3)24 were also observed, which are common to all laser-ablated metal experiments with CCl4 due to ablation plume photolysis. The product absorptions increased in concert 10%, 0%, and 20% on sequential irradiations in the visible (λ > 420 nm), ultraviolet (240-380 nm), and full arc (λ > 220 nm) regions, respectively. A similar experiment with 13CCl4 (90% enriched) shifted the new absorptions to 930.5 cm-1 (with a satellite at 924.7 cm-1) (12/13 isotopic frequency ratio of 1.0363) and to 840.2 cm-1 (with a shoulder at 837.9 cm-1) (12/13 ratio of 1.0321). The appearance of a weak 12C product band at 964.3 cm-1 with about 1/10 of the 13C product band absorbance (Figure 1) reflects single carbon atom participation in these vibrational modes. The 865.4 and 837.9 cm-1 shoulders on the 867.2 and 840.2 cm-1 bands are 6/9 of the main band absorbances, which are appropriate for the statistical ratio of natural abundance for two equivalent chlorine atoms (one 35Cl and one 37Cl vs two 35Cl atoms). The two strong m product absorptions in the C-Cl stretching region suggest that a primary product with a CCl2 moiety is generated during co-deposition of the laserablated rhodium atoms and CCl4. On the basis of our previous experience for reactions of metal atoms with hydrocarbons and halomethanes,4-9 they are assigned to the symmetric and antisymmetric CCl2 stretching modes of CCl2dRhCl2. Mixing of the symmetric CCl2 stretching with C-Rh stretching modes leads to the higher symmetric stretching frequency and increases the carbon participation and the 12/13 isotopic frequency ratio, as found for group 10 metal methylidenes.8a-c In this mode, C vibrates back and (22) (a) Jacox, M. E.; Milligan, D. E. J. Chem. Phys. 1971, 54, 3935. (b) Jacox, M. E. Chem. Phys. 1976, 12, 51. (c) Prochaska, F. T.; Andrews, L. J. Chem. Phys. 1977, 67, 1091 (CCl3þ). (23) Maier, G.; Reisenauer, H. P.; Hu, J.; Hess, B. A., Jr.; Schaad, L. J. Tetrahedron Lett. 1989, 30, 4105 (Cl2CCl-Cl). (24) Andrews, L. J. Chem. Phys. 1968, 48, 972 (CCl3).

Figure 3. Infrared spectra in the 1200-600 cm-1 region for the reaction products of the laser-ablated rhodium atom with CHCl3 isotopomers in excess argon at 10 K. (a) Rh and CHCl3 (0.5% in argon) co-deposited for 1 h; (b-e) as (a) spectra taken following the irradiation and annealing sequence described in Figure 1 caption (visible, UV, and full arc irradiations and annealing to 28 K); (f) Rh and CDCl3 reagent (0.5% in argon) co-deposited for 1 h; (g-j) as (f) spectra taken following a sequence of visible and UV irradiation and annealing to 28 K.; (k) Rh and 13CHCl3 reagent (0.5% in argon) co-deposited for 1 h, (l-o) as (k) spectra taken following a sequence of visible and UV irradiation and annealing to 28 K. m designates a product absorption, while P and c stand for the precursor and common absorptions.

forth between Rh and two Cl atoms in an antisymmetric fashion. The totally symmetric counterpart predicted at 437.8 cm-1 (Table 1A), on the other hand, has a very small carbon13 shift (0.4 cm-1), as the carbon atom hardly moves. The 867.2 cm-1 band is due to the antisymmetric CCl2 stretching mode, and the 1.0321 isotopic 12/13 frequency ratio is virtually the same as found for the group 10 metal methylidenes, CCl2-MCl2.8a-c The m absorption at 410.7 cm-1 near our observation limit shows a very small 13C shift of 1.0 cm-1 and is assigned to the RhCl2 antisymmetric stretching mode. The observed symmetric and antisymmetric CCl2 stretching and antisymmetric RhCl2 frequencies correlate well with the predicted values for staggered CCl2-RhCl2 in its doublet ground state, as shown in Table 1A, although both density functional computed values fall below the observed frequencies, which is not the typical result,19,20 presumably because DFT underestimates the rhodium bond strengths. However, the carbon-13 isotopic shifts for the modes are well reproduced (observed 33.8, 27.0, and 1.0 cm-1 vs predicted 34.2, 27.1, and 0.7 cm-1). The observed CCl2 stretching

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Figure 4. Infrared spectra in the 1400-800 cm-1 region for the reaction products of the laser-ablated rhodium atom with CH2Cl2 isotopomers in excess argon at 10 K. (a) Rh and CH2Cl2 (0.5% in argon) co-deposited for 1 h; (b-e) as (a) spectra taken following a sequence of visible, UV, and visible irradiations and annealing to 28 K; (f) Rh and CD2Cl2 reagent (0.5% in argon) co-deposited for 1 h; (g) as (f) after visible (λ > 420 nm); (h) as (g) after annealing to 38 K; (i) Rh and 13CH2Cl2 reagent (0.5% in argon) co-deposited for 1 h; (j-m) as (i) spectra taken following the irradiation and annealing sequence described in Figure 1 caption (visible, UV, and full arc irradiations and annealing to 28 K). m designates a product absorption, while P and c stand for the precursor and common absorptions.

frequencies are, however, slightly higher than the predicted values, similar to those observed from the previously studied halogenated late transition-metal carbene and carbynes.7,8 Unfortunately all of the other vibrational bands of the Rh methylidene are too weak to observe (Table 1A). The three observed strong (symmetric and antisymmetric CCl2 and antisymmetric RhCl2 stretching) absorptions and their 13C counterparts, which are consistent with the DFT results, substantiate formation of the Rh methylidene, staggered CCl2dRhCl2, via C-Cl bond insertion by Rh and subsequent Cl migration from C to Rh. (In contrast, the planar CCl2dRhCl2 isomer is 6 kcal/mol higher in energy, and its two strongest computed frequencies, 907 and 822 cm-1, are considerably lower than the observed values.) Another product absorption and its 13C counterpart marked “y” are also observed at 1118.4 and 1080.6 cm-1, which almost disappear on visible irradiation but dramatically increase (more than 500% of the original intensity) on UV irradiation and slightly decrease on full arc photolysis. Their relatively high frequencies suggest that they originate from a higher-oxidation-state complex, mostly likely a Rh carbyne complex (ClCtRhCl3).4-7 The observed frequencies are compared with computed Cl-C-Rh antisymmetric stretching frequencies of 1140.3 and 1100.7 cm-1 predicted for ClCtRhCl3 in its doublet ground state (Table 1B), which is 26 kcal/mol higher than CCl2dRhCl2(doublet). The excellent agreement of these unique observed frequencies and 13C shift support formation of the rhodium carbyne complex.

Cho and Andrews

Figure 5. Infrared spectra in the 1300-900 and 650-550 cm-1 region for the reaction products of the laser-ablated rhodium atom with CH2FCl isotopomers in excess argon at 10 K. (a) Rh and CH2FCl (0.5% in argon) co-deposited for 1 h; (b-e) as (a) spectra taken following a sequence of visible, UV, and visible irradiations and annealing to 28 K; (f) Rh and CD2FCl reagent (0.5% in argon) co-deposited for 1 h; (g-j) as (f) spectra taken following the same sequence. i and m designate the product absorption group, while P and c stand for the precursor and common absorptions.

Figure 6. Infrared spectra in the 1000-500 cm-1 region for the reaction products of the laser-ablated rhodium atom with CH2F2 isotopomers in excess argon at 10 K. (a) Rh and CH2F2 (0.5% in argon) co-deposited for 1 h; (b-e) as (a) spectra taken following the irradiation and annealing sequence described in Figure 1 caption (visible, UV, and full arc irradiations and annealing to 28 K); (f) Rh and CD2F2 reagent (0.5% in argon) co-deposited for 1 h; (g-j) as (f) spectra taken following the same sequence. m designates a product absorption, while P and c stand for the precursor and common precursor product absorptions. The RhO2 absorption27 is also shown.

The distinctively high Cl-C-M antisymmetric stretching frequency, originating from the carbon atom moving back and forth between the halogen and metal atoms, provides strong evidence for formation of the methylidyne complex. However, it is unfortunate that, as Table 1B shows, the other

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Figure 7. Infrared spectra in the 2250-1850, 1450-850, and 650-450 cm-1 region for the reaction products of the laserablated rhodium atom with CH3F isotopomers in excess argon at 10 K. (a) Rh and CH3F (0.5% in argon) co-deposited for 1 h; (b-d) as (a) spectra taken following a sequence of λ > 290 nm and UV irradiations and annealing to 28 K; (e) Rh and CD3F reagent (0.5% in argon) co-deposited for 1 h; (f) as (e) after UV irradiation. i designates a product absorption, while P stands for a precursor absorption. Weak RhNN and RhCO absorptions are also shown.27,28.

bands from ClCtRhCl3 are all too weak to observe. It is worth mentioning at this point that Re and Os exclusively form carbyne complexes in reaction with small hydrocarbons and halomethanes, while Ru mostly produces carbynes as well as carbenes.6,7 Pd, on the other hand, generates small carbene and insertion products in reaction with tetra-halomethanes, while it produces only insertion complexes with other halomethanes and alkanes.8 Rh, located between Ru and Pd, forms both methylidene and methylidyne complexes, showing a gradual decrease in preference for the higher-oxidation-state product. In contrast, the insertion complex, CCl3-RhCl, is not identified in the matrix spectra. (Table S1 gives the computed frequencies for this unobserved species.) The observed spectra are basically consistent with the DFT computations; ClCt RhCl3, CCl2dRhCl2, and CCl3-RhCl in their doublet ground states are 59, 85, and 70 kcal/mol more stable than the reactants (Rh(4F) þ CCl4). This suggests that CCl3-RhCl, after forming via C-Cl bond insertion, directly undergoes Cl migration from C to Rh, producing the more stable primary product, staggered CCl2dRhCl2. Then ClCtRhCl3 is mostly formed on UV irradiation afterward, and it contrasts the previous results that Ru produces mostly carbynes during co-deposition with halomethanes as well as in the process of photolysis afterward.8 The present results, thus, reconfirm the general trend that the higher-oxidation-state complexes become less favored on moving away from W, Re, and Os. Rh þ CFCl3 and CF2Cl2. Figure 2 shows infrared spectra from reactions of Rh atoms with two chlorofluoromethanes. In the CFCl3 spectra three m absorptions are observed, which increase in concert ∼15%, ∼30%, and ∼25% (∼70% in total) on visible, UV, and full arc irradiations. The two strong product absorptions, one at 1214.6 cm-1 over a common absorption in the C-F stretching region21 and the other one at 963.3 cm-1 (with a shoulder at


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Figure 8. Infrared spectra in the 2100-1800 and 1550850 cm-1 region for the reaction products of the laser-ablated rhodium atom with ethane isotopomers in excess argon at 10 K. (a) Rh and C2H6 (0.5% in argon) co-deposited for 1 h; (b-e) as (a) spectra taken following a sequence of visible, UV, and full arc irradiations and annealing to 28 K.; (f) Rh and C2D6 reagent (0.5% in argon) co-deposited for 1 h; (g-i) as (f) spectra taken following a sequence of visible and UV irradiations and annealing to 28K. i designates a product absorption while P and c stand for the precursor and common absorptions.

956.8 cm-1) in the C-Cl stretching region,21 suggest a primary product with a CFCl moiety. It is most probably another small Rh methylidene, CFCl-RhCl2, and the observed C-F and C-Cl stretching frequencies correlate well with the DFT values (e.g., B3LYP frequencies of 1223.6 and 963.3 cm-1), as shown in Table 2A. The weak absorption at 415 cm-1 is assigned to the antisymmetric RhCl2 stretching mode. The other bands are all too weak to observe, as shown in Table 2A. The good agreement between the observed and predicted values supports formation of the F-containing Rh methylidene, CFCldRhCl2. Another product absorption marked “y” is observed at 1482.0 cm-1, which increases ∼20% on visible irradiation and slightly decreases on UV and full arc photolysis. Its remarkably high frequency again most probably originates from the F-C-Rh antisymmetric stretching mode of FCt RhCl3, in line with the Ru þ CFCl3 case.7 The observed frequency also correlates with the predicted 1458.1 cm-1 as shown in Table 2B, and it is the uniquely strong band of the Rh carbyne, while the other FCtRhCl3 absorptions are too weak to observe. On the other hand, the insertion complex is not identified, parallel to the Rh þ CCl4 case. While FCtRhCl3, staggered CFCldRhCl2, and CFCl2RhCl are 49, 79, and 66 kcal/mol more stable than the reactants, a higher-oxidation-state complex is normally expected to be more stabilized in the matrix due to its more polarized bonds.4-8 In this case the planar CFCldRhCl2 complex is 3 kcal/mol higher in energy than the staggered form. In the CF2Cl2 spectra the three m absorptions are observed, which remain unchanged on visible irradiation but increase ∼30% and another ∼20% on UV and full arc photolysis. The two strong m absorptions at 1283.0 cm-1 (with a shoulder at 1284.9 cm-1) and 1251.3 cm-1 in the C-F stretching region21 reveal formation of a primary product

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Table 1A. Observed and Calculated Fundamental Frequencies of CCl2dRhCl2 in the Ground 2A2 Statea 13

CCl2dRhCl2

approximate description

b

A1 CCl2 s. str. B2 CCl2 as. str. B1 CCl2Rh deform A1 C-Rh str. B1 RhCl2 as. str. A1 RhCl2 s. str. A1 CCl2 scis. B2 CCl2 rock A1 RhCl2 scis. B1 RhCl2 rock B2 RhCl2 wag A2 CCl2 tort.

CCl2dRhCl2

obs

B3LYP

int

BPW91

int

obs

B3LYPc

intc

BPW91d

intd

964.3, 958.6 867.2, 865.4

952.8 844.8 470.1 437.8 395.5 331.6 236.0 196.7 105.9 76.5 66.3 22.0

257 197 3 5 96 13 1 1 0 0 3 0

945.0 802.2 455.9 428.6 397.3 340.1 232.2 192.7 105.2 76.7 63.3 17.4

236 186 6 8 69 9 1 0 0 0 1 0

930.5, 924.7 840.2, 837.9

918.6 817.7 453.6 437.4 394.8 331.5 236.0 195.8 105.9 76.5 66.3 22.0

239 184 5 5 94 13 1 1 0 0 3 0

911.0 776.5 440.6 428.3 396.0 340.0 232.2 191.9 105.1 76.7 63.3 17.4

219 174 10 8 65 9 1 0 0 0 1 0

410.7, 407.4

c

c

d

d

b

409.7, 406.0

a Frequencies and intensities are in cm-1 and km/mol. CCl2dRhCl2(D) has a staggered C2v structure. The symmetry notations are based on the C2v structures. b Observed in an argon matrix. The stronger matrix site absorption is given in bold type. c Frequencies and intensities computed with B3LYP/ 6-311þþG(3df, 3pd). d Frequencies and intensities computed with BPW91/6-311þþG(3df,3pd).

Table 1B. Observed and Calculated Fundamental Frequencies of ClCtRhCl3 Isotopomers in the Ground 2A0 Statesa Cl13CtRhCl3

ClCtRhCl3

approximate description A0 ClCRh as. str. A0 ClCRh s. str. A00 RhCl2 as. str. A0 Rh-Cl str. A0 RhCl2 s. str. A00 ClCRh bend A0 ClCRh bend A0 RhCl2 wag A0 ClCRh bend A00 RhCl3 deform A0 CRhCl bend A00 C-Cl tort.

obsb

B3LYPc

intc

BPW91d

intd

obsb

B3LYPc

intc

BPW91d

intd

1118.4

1140.3 539.3 376.5 362.0 334.5 308.2 297.3 133.5 108.0 99.5 50.8 40.6

344 27 93 12 8 0 5 0 0 1 2 0

1151.0 509.4 372.0 353.3 326.8 307.8 289.1 132.5 97.5 94.4 48.2 38.4

288 40 81 9 9 0 8 0 1 2 2 0

1080.6

1100.7 527.5 376.5 361.9 334.1 297.2 293.4 133.4 108.0 99.4 50.8 40.6

321 25 93 13 8 0 5 0 0 1 2 0

1110.7 499.7 372.0 352.9 326.6 296.9 284.6 132.4 97.4 94.4 48.2 38.4

269 37 81 9 9 0 9 0 1 2 2 0

a Frequencies and intensities are in cm-1 and km/mol. ClCtRhCl3(D) has a Cs structure. The symmetry notations are based on the Cs structure. Observed in an argon matrix. c Frequencies computed with B3LYP/6-311þþG(3df, 3pd). d Frequencies and intensities computed with BPW91/ 6-311þþG(3df,3pd). b

Table 2A. Observed and Calculated Fundamental Frequencies of CF2dRhCl2 and CFCldRhCl2 in Their Ground 2A and 2A00 Statesa CF2dRhCl2

approximate description A CF2 s. str. B CF2 as. str. A CCl2 scis. B CF2 wag B RhCl2 as. str. A C-Rh str. A RhCl2 s. str. B CF2 rock A RhCl2 scis. B RhCl2 rock B RhCl2 wag A CF2 tort.

obs

B3LYP

int

BPW91

int


1284.9, 1283.0 1251.3

1296.1 1247.2 719.3 573.4 405.9 383.3 339.7 296.6 121.6 96.1 70.8 41.2

622 216 6 3 101 2 2 6 1 0 4 0

1246.8 1160.5 691.3 559.8 404.2 387.7 340.4 290.6 116.4 88.3 74.3 18.6

588 195 1 2 75 4 1 2 0 0 2 0

A0 C-F str. A0 C-Cl str. A0 CFCl wag A00 FClCRh deform A00 RhCl2 as. str. A0 C-Rh str. A0 RhCl2 s. str. A0 CFCl rock A0 RhCl2 scis. A00 RhCl2 rock A0 RhCl2 wag A00 CFCl2 tort.

b

417

c

c

d

d

CFCl=RhCl2 b

obs

B3LYPc

intc

BPW91d

intd

1214.0 963.3, 956.8

1223.6 950.3 533.6 522.2 400.3 348.8 331.6 217.1 110.6 82.4 67.6 26.3

327 331 3 1 101 9 3 1 1 0 3 0

1168.0 912.8 518.2 505.8 400.7 351.1 332.0 213.2 109.2 82.6 67.2 10.1

324 276 8 0 76 10 0 0 0 0 1 0

415

a Frequencies and intensities are in cm-1 and km/mol. CF2dRhCl2(D) and CFCldRhCl2(D) have staggered C2 and Cs structures, and the symmetry notations are based on the structures. b Observed in an argon matrix. The stronger matrix site absorption is given in bold type. c Frequencies and intensities computed with B3LYP/6-311þþG(3df, 3pd). d Frequencies and intensities computed with BPW91/6-311þþG(3df,3pd).

with a CF2 group, most probably CF2dRhCl2. The observed CF2 stretching bands are assigned to the symmetric and antisymmetric modes of the Rh methylidene, and the frequencies are well reproduced, as shown in Table 2A. A weak m absorption is also observed at 417 cm-1 and assigned to the RhCl2 antisymmetric stretching mode.

Unlike the CCl4 and CFCl3 cases, the carbyne complex (FCtRhFCl2) is less likely due to its much higher energy; FCtRhFCl2 and staggered CF2dRhCl2 are 25 and 75 kcal/ mol more stable than the reactants. However, a weak absorption at 1495.0 cm-1 triples on UV and full arc photolysis, which is also compared with the predicted F-C-Rh

Article


2217

Table 2B. Observed and Calculated Fundamental Frequencies of FCtRhCl3 and FCtRhFCl2 in the Ground 2A0 and 2A Statesa FCtRhCl3

approximate description A0 FCRh as. str. A0 FCRh s. str. A0 ClCRh bend A00 RhCl2 as. str. A0 Rh-Cl str. A0 RhCl2 s. str. A00 FCRh bend A0 RhCl2 wag A0 RhCl3 deform A00 RhCl3 deform A0 RhCl2 scis. A00 C-F tort.

FCtRhFCl2

obs

B3LYP

int

BPW91

int


1482.0

1458.1 663.1 391.6 376.6 357.1 333.1 316.8 136.3 105.5 99.2 62.6 42.6

624 2 1 99 16 4 0 0 2 1 1 0

1416.3 642.8 380.3 370.3 344.1 328.2 315.9 135.9 103.6 85.5 56.0 12.2

555 12 1 84 17 4 0 0 1 3 1 1

FCRh as. str. FCRh s. str. Rh-F str. FCRh bend RhCl2 s. str. RhCl2 as. str. FCRh oop bend RhFCl wag RhFCl2 deform RhFCl2 deform CRhCl bend C-F tort.

b

c

c

d

d

b

obs

B3LYPc

intc

BPW91d

intd

1495.0e

1463.5 666.6 584.0 393.5 365.4 361.8 321.5 181.1 139.7 105.4 71.9 45.0

586 11 125 3 20 27 2 2 4 1 1 0

1453.7 615.2 580.6 431.9 358.2 334.2 407.6 195.6 149.1 122.3 74.4 35.2

570 15 77 10 10 23 4 1 0 4 2 2

588.9e

a Frequencies and intensities are in cm-1 and km/mol. FCtRhCl3(D) and FCtRhFCl2(D) have Cs and C1 structures. The symmetry notations are based on the Cs structure. b Observed in an argon matrix. c Frequencies and intensities computed with B3LYP/6-311þþG(3df, 3pd). d Frequencies and intensities computed with BPW91/6-311þþG(3df,3pd). e Tentative assignment.

Table 3. Observed and Calculated Fundamental Frequencies of CHCldRhCl2 Isotopomers in the Ground 2A00 Statea CHCldRhCl2


obsb

B3LYPc

A0 C-H str.

intc

3110.1

A0 HCRh bend A0 C-Cl str. A00 C-H OOP bend A0 C-Rh str. A00 RhCl2 as. str. A0 RhCl2 s. str. A0 ClCRh bend A00 HCClRh deform A0 RhCl2 bend A0 RhCl2 wag A00 CHCl twist

1163.3, 1159.9, 1154.8 907.8 721.7 659.7

7

13

CDCldRhCl2

BPW91d

intd

3039.8

obsb

5

1206.8

73

1148.2

70

908.5 749.8 660.2 395.1 333.4 227.1 169.4 104.2 64.6 27.1

208 11 14 97 12 1 4 1 3 0

890.9 702.2 652.9 398.6 341.2 222.2 187.6 108.8 66.0 39.7

181 14 26 71 9 0 5 0 1 0

B3LYPc

intc

2288.2 997.0, 995.6, 993.8 783.3

CHCldRhCl2

BPW91d

4

intd

2236.1

obsb

3

1008.9

220

972.6

206

798.7 627.3 619.4 395.0 333.3 225.3 149.1 103.9 64.6 26.6

64 5 7 97 12 1 3 1 3 0

771.1 617.9 585.6 398.3 341.1 220.9 163.4 108.7 66.0 38.9

50 16 7 71 9 0 3 0 1 0

1154.2, 1150.2, 1145.0 884.0 e e

B3LYPc

intc

BPW91d

intd

3100.6

7

3030.6

5

1198.2

60

1140.1

57

882.9 739.3 643.3 395.0 333.3 225.5 166.2 104.1 64.6 27.1

204 12 13 97 12 1 4 1 3 0

865.5 692.4 636.5 398.6 341.1 220.7 183.8 108.7 66.0 39.6

178 15 25 70 9 0 4 0 1 0

a Frequencies and intensities are in cm-1 and km/mol. CHCldRhCl2 has a staggered Cs structure with two equal Rh-Cl bonds. The symmetry notations are based on the Cs structure. b Observed in an argon matrix. The strongest matrix site absorption is given in bold type. c Frequencies computed with B3LYP/6-311þþG(3df, 3pd). d Frequencies and intensities computed with BPW91/6-311þþG(3df,3pd). e Covered by precursor absorption.

Table 4. Observed and Calculated Fundamental Frequencies of CH2dRhCl2 Isotopomers in the Ground 2A2 Statea approximate description B1 as. CH2 str. A1 s. CH2 str. A1 CH2 scis. B2 CH2 wag A1 C-Rh str. B1 CH2 rock B2 as. RhCl2 str. A1 s. RhCl2 str. A2 CH2 tort B2 RhCl2 rock A1 RhCl2 scis. B1 RhCl2 wag

CH2dRhCl2 b

obs

1311.2 878.1

c

c

13

CD2dRhCl2 d

d

B3LYP

int

BPW91

int

3178.8 3049.5 1375.9 937.4 796.8 755.4 398.8 343.5 190.5 130.6 113.7 75.6

0 0 8 16 2 6 99 8 5 0 0 0

3102.1 2972.4 1321.1 882.2 799.6 730.1 403.5 347.9 261.3 197.0 116.2 80.1

0 1 11 20 0 5 74 6 0 4 0 0

b

obs

1037.4 e

c

c

CH2dRhCl2

d

d

B3LYP

int

BPW91

int

2369.3 2201.8 1084.9 744.2 706.0 567.9 398.7 343.4 173.1 113.5 92.7 73.7

0 0 7 7 0 2 99 8 4 0 0 1

2312.5 2144.3 1047.5 700.4 705.2 548.6 403.2 347.8 185.3 179.1 115.9 78.0

0 1 7 10 0 2 74 6 0 30 0 0

b

obs

1302.8 870.3

B3LYPc

intc

BPW91d

intd

3165.4 3045.1 1366.2 928.3 775.3 751.3 398.8 343.5 186.0 130.6 113.6 75.2

0 0 7 17 2 6 99 8 5 0 0 0

3089.0 2968.2 1311.7 873.6 778.0 726.2 403.5 347.8 261.3 192.5 116.0 79.7

0 1 10 20 0 5 74 6 0 4 0 0

a Frequencies and intensities are in cm-1 and km/mol. CH2dRhCl2 has a staggered C2v structure, and the symmetry notations are based on the C2v structure. b Observed in an argon matrix. c Frequencies computed with B3LYP/6-311þþG(3df, 3pd). d Frequencies and intensities computed with BPW91/6-311þþG(3df,3pd). e Covered by precursor band.

antisymmetric stretching frequency of 1463.5 cm-1 (Table 2A), leaving a possibility that a small amount of the less stable difluoro Rh carbyne is also produced on UV irradiation. Again the insertion complex, CF2Cl-RhCl, is

not identified in the spectra; attempts for geometry optimization of CF2Cl-RhCl all end up with the structure of CF2dRhCl2. This suggests that C-Cl bond insertion of the chlorofluorocarbon directly leads to the primary

2218


Cho and Andrews

Table 5. Observed and Calculated Fundamental Frequencies of CH2dRhFCl Isotopomers in the Ground 2A00 Statea CH2dRhFCl


b

c

obs

A00 as. CH2 str. A0 s. CH2 str. A0 CH2 scis. A0 CH2 wag A0 C-Rh str. A00 CH2 rock A0 Rh-F str. A0 Rh-Cl str. A00 FRhCl rock A0 CH2 twist A0 RhFCl bend A00 RhFCl wag

928.0 591.3

CD2dRhFCl

B3LYP

c

d

d

int

BPW91

int

3182.1 3055.6 1375.5 942.4 787.8 761.7 600.8 374.3 201.4 151.9 142.7 32.7

0 0 7 26 1 6 130 32 5 1 1 4

3107.1 2977.6 1321.4 888.6 796.5 738.5 594.3 379.1 275.9 206.1 145.4 97.1

0 1 10 28 0 6 103 23 0 4 1 2

b

c

obs

e

590.8

B3LYP

intc

BPW91d

intd

2371.7 2204.8 1081.9 751.5 699.3 573.3 600.0 374.2 184.6 144.0 125.7 27.2

0 0 5 16 0 3 127 32 4 2 1 3

2316.2 2148.3 1046.8 711.9 699.3 555.7 593.3 379.0 198.5 189.1 144.4 93.9

0 1 6 12 8 3 99 23 0 3 1 2

a Frequencies and intensities are in cm-1 and km/mol. CH2dRhFCl has a staggered Cs structure, and the symmetry notations are based on the Cs structure. b Observed in an argon matrix. c Frequencies computed with B3LYP/6-311þþG(3df, 3pd). d Frequencies and intensities computed with BPW91/6-311þþG(3df,3pd). e Covered by precursor band.

Table 6. Observed and Calculated Fundamental Frequencies of CH2F-RhCl Isotopomers in the 2A0 Statea approximate description A00 CH2 as. str. A0 CH2 s. str. A0 CH2 scis. A0 CH2 wag A00 CH2 twist A0 C-F str. A00 CH2 rock A0 C-Rh str. A0 Rh-Cl str. A0 FCRh bend A00 CH2 rock A0 CRhCl bend

CH2F-RhCl obsb

1217.9 970.5, 967.7

CD2F-RhCl

B3LYPc

intc

BPW91d

intd

3133.3 3045.6 1466.5 1242.0 1174.0 943.8 712.6 621.4 367.4 217.8 78.0 66.8

6 12 3 53 3 199 3 14 36 7 7 11

3052.7 2968.0 1412.9 1196.1 1127.7 895.6 673.8 616.0 372.2 216.2 75.4 65.6

4 14 4 47 0 177 22 24 28 6 6 10

obsb

954.3 942.6, 940.8

B3LYPc

intc

BPW91d

intd

2332.7 2201.2 1090.4 974.6 882.2 919.0 534.2 561.5 368.8 218.8 71.6 78.8

4 10 9 59 2 178 1 5 36 7 6 11

2272.0 2144.0 1049.3 937.8 847.2 873.2 505.4 556.8 372.2 214.6 70.0 64.8

2 11 4 46 0 177 11 10 27 6 5 9

a Frequencies and intensities are in cm-1 and km/mol. CH2F-RhCl has a Cs structure, and the symmetry notations are based on the Cs structure. Observed in an argon matrix. The stronger matrix site absorption is given in bold type. c Frequencies computed with B3LYP/6-311þþG(3df, 3pd). d Frequencies and intensities computed with BPW91/6-311þþG(3df,3pd). b

Table 7. Observed and Calculated Fundamental Frequencies of CH2dRhF2 Isotopomers in the 2A2 Ground Statea approximate description B1 CH2 as. str. A1 CH2 s. str. A1 CH2 scis. B2 CH2 wag. A1 C-Rh str. B1 CH2 rock B2 RhF2 as. str. A1 RhF2 s. str. B2 RhF2 rock A1 RhF2 bend B1 RhF2 wag A2 CH2 tort.

CH2dRhF2 b

obs

911.5, 909.6 734.4 637.9, 634.9

c

CD2dRhF2

B3LYP

int

c

d

d

BPW91

int

3185.8 3057.3 1372.4 945.4 770.8 769.3 647.0 577.5 213.4 185.0 137.2 113.5

0 0 3 33 0 10 167 37 5 5 7 0

3109.2 2980.0 1321.5 896.7 791.1 749.0 640.2 570.5 220.7 184.7 135.6 268.7

0 1 7 35 0 9 131 31 4 3 4 0

b

obs

e

635.8, 632.8 567.8

B3LYPc

intc

BPW91d

intd

2374.4 2207.4 1074.8 759.0 687.5 579.7 645.2 577.5 195.0 184.8 135.3 80.4

0 0 2 29 0 4 158 37 4 5 7 0

2317.8 2150.3 1045.2 720.0 699.3 564.1 637.7 570.5 202.0 184.4 133.5 191.2

0 1 4 35 0 4 117 31 3 3 4 0

a Frequencies and intensities are in cm-1 and km/mol. CH2dRhF2 has a staggered C2v structure, and the symmetry notations are based on the C2v structure. b Observed in an argon matrix. The stronger matrix site absorption is given in bold type. c Frequencies computed with B3LYP/6-311þþG(3df, 3pd). d Frequencies and intensities computed with BPW91/6-311þþG(3df,3pd). e Covered by precursor band.

product, staggered CF2dRhCl2, which is 1 kcal/mol lower in energy than the planar form. The Rh methylidene with a Rh-F bond is, on the other hand, far less favored (for example, CFCldRhFCl is 25 kcal/mol higher in energy than CF2dRhCl2). This is consistent with the fact that the Rh-F stretching absorptions, which would be very intense and

expected near 570 cm-1, are not observed in the CFCl3 and CF2Cl2 spectra. Rh þ CHCl3. The product absorptions in the infrared spectra from reactions of Rh with CHCl3 isotopomers are shown in Figure 3. The product absorptions (all marked with “m”), which are weaker relative to the tetra-halomethane

Article


2219

Table 8. Calculated Fundamental Frequencies of CH3-RhF Isotopomers in the Ground 2A0 Statea CH3-RhF

approximate description A00 CH3 as. str. A0 CH3 as. str. A0 CH3 s. str. A0 CH3 bend A00 CH3 bend A0 CH3 deform A0 CH3 rock A00 CH3 rock A0 Rh-F str. A0 C-Rh str. A00 CH3 tort A0 CRhF bend

b

obs e e

1183.1 559.8, 550.5

c

CD3-RhF

c

d

d

B3LYP

int

BPW91

int

3115.3 3072.1 2947.5 1453.8 1408.9 1250.7 717.6 702.0 582.9 547.4 127.3 114.8

9 22 23 2 3 31 4 7 92 5 1 10

3053.3 3006.5 2861.0 1405.6 1346.8 1193.0 691.5 664.7 580.6 534.2 145.5 99.4

8 24 23 2 2 30 2 6 77 6 1 10

b

obs

B3LYPc

intc

BPW91d

intd

2309.1 2262.6 2116.5 1059.0 1026.9 963.3 562.0 520.1 583.1 485.0 95.0 106.7

3 14 9 2 2 13 1 4 93 6 1 11

2262.9 2210.3 2056.5 1024.7 982.5 928.1 556.0 491.9 580.5 463.6 104.5 91.3

3 16 9 2 2 14 5 3 73 5 1 10

2199.5 e

920.5 552.6, 546.1

a Frequencies and intensities are in cm-1 and km/mol. CH3RhF has a Cs structure, and the symmetry notations are based on the Cs structure. Observed in an argon matrix. The stronger matrix site absorption is given in bold type. c Frequencies computed with B3LYP/6-311þþG(3df, 3pd). d Frequencies and intensities computed with BPW91/6-311þþG(3df,3pd). e Covered by product absorption. b

Table 9. Observed and Calculated Fundamental Frequencies of CH2CH3-RhH Isotopomers in the Ground 2A Statea approximate description CH3 as. str. CH3 as. str. CH3 s. str. CH2 s. str. Rh-H str. CH2 wag CRhH bend

CH3CH2-RhH(D) b

obs

2052.9, 2045.8, 2030.9 1183.9, 1168.3, 1160.0

c

CD3CD2-RhD(D)

B3LYP

c

d

d

int

BPW91

int

3098.5 3056.4 2994.4 2925.8 2122.1 1206.3 436.5

23 28 39 43 120 92 61

3051.7 3007.1 2937.1 2859.5 2136.2 1160.9 412.9

19 20 36 43 71 91 55

b

obs

1474.6, 1471.3, 1461.5 952.3, 946.1

B3LYPc

intc

BPW91d

intd

2294.3 2256.0 2157.8 2129.3 1508.5 991.6 324.1

11 12 14 21 61 55 36

2259.5 2218.2 2116.5 2080.1 1518.5 956.2 303.6

9 9 13 21 36 56 31

a Frequencies and intensities are in cm-1 and km/mol. C2H5-RhH has a C1 structure. Only observable bands are listed: The frequencies are higher than 400 cm-1 and intensities higher than 20 km/mol. b Observed in an argon matrix. Strongest peaks are in bold. c Frequencies computed with B3LYP/ 6-311þþG(3df, 3pd). d Frequencies and intensities computed with BPW91/6-311þþG(3df,3pd).

cases, increase ∼30% on visible irradiation; however, they almost disappear during the following UV photolysis. They slightly recover during the following full arc irradiation and sharpen in the early stage of annealing. Therefore, the relatively weak absorptions in the original spectra are believed to be at least partly due to precursor dissociation by ablation plume UV radiation. The major product absorptions are assigned to the methylidene complex, CHCld RhCl2. The m absorption at 1159.9 cm-1 (with site absorptions at 1163.3 and 1154.8 cm-1) has its D counterpart at 995.6 cm-1 (with site absorptions at 997.0 and 993.8 cm-1) (H/D ratio of 1.165) and its 13C counterpart at 1150.2 cm-1 (with site absorptions at 1154.2 and 1145.0 cm-1) (12/13 ratio of 1.008). It is assigned to the HCRh bending mode on the basis of the relatively large D and small 13C shifts. The m absorption at 907.8 cm-1 has its D counterpart at 783.3 cm-1 (H/D ratio of 1.159) partly overlapped by a common absorption and 13C counterpart at 884 cm-1 (12/13 ratio of 1.027). It is due to the C-Cl stretching mode incorporated with the C-Rh stretching mode, where as a result, the C atom (with H) moves back and forth between the Cl and Rh atoms, leading to a relatively large 12/13 ratio. Two weak m absorptions are observed at 721.7 and 659.7 cm-1 in the CHCl3 spectra, and they are assigned to the C-H out-of-plane bending and C-Rh stretching modes without observation of the D and 13C counterparts. The other bands are either too weak to observe or beyond our observation range. The observed m absorptions in good agreement with the DFT values, as shown in Table 3, substantiate the formation of CHCldRhCl2. In contrast, the methylidyne

and insertion complexes (HCtRhCl3 and CHCl2-RhCl) are not identified in the spectra. In this case HCtRhCl3, CHCldRhCl2, and CHCl2-RhCl are 35, 72, and 63 kcal/ mol more stable than the reactants. Rh þ CH2X2. Figures 4-6 illustrate spectra from reaction products of Rh with CH2Cl2, CH2FCl, and CH2F2, and parallel to the tetrahalomethane and CHCl3 cases, Rh methylidenes are the primary products. Figure 4 shows infrared spectra from reactions with CH2Cl2 isotopomers, where the m absorptions increase ∼20% on visible irradiation but decrease to ∼25% of the original intensity on UV irradiation and slightly recover on subsequent visible irradiation. The relatively weak product (m) absorptions most probably result from plume UV radiation during co-deposition. The m absorption at 1311.2 cm-1 has its D and 13C counterparts at 1037.4 and 1302.8 cm-1 (H/D and 12/13 ratios of 1.264 and 1.006) and are assigned to the CH2 scissoring mode of CH2dRhCl2 on the basis of the frequency and relatively large and small D and 13C shifts. Another m absorption at 878.1 cm-1 has its 13C counterpart at 870.3 cm-1 (12/13 ratio of 1.009), while the D counterpart is believed to be covered by precursor absorption. It is designated as the CH2 wagging mode on the basis of the frequency, small 13C shift, and DFT values. However, the observably strong CH2 rocking band is covered by precursor absorption, and the other bands are predicted to be too weak to observe or beyond our observation range. The observed frequencies are about 95 and 99% the B3LYP and BPW91 values, and the H/D and 12/13 ratios are also well reproduced (e.g., H/D ratio of 1.264 vs 1.268 for

2220


Figure 9. Structures calculated for the reaction products of rhodium with halomethanes and ethane at the B3LYP level of theory using the 6-311þþG(3df,3pd) basis sets for H, C, F, and Cl. Bond distances and angles are in A˚ and deg. The tetra-halo- and trihalomethylidenes have staggered allene-type structures, consistent with their stabilities over the planar structures. The observed vibrational characteristics show that the di- and mono-halomethanes also have allene-type structures, although their energies are comparable with those of planar structure. The identified tetra-halomethylidynes, ClCtRhCl3 and FCtRhCl3, have Cs structures. Molecular symmetries are given under each structure.

the CH2 scissoring band). This good agreement substantiates formation of CH2dRhCl2 in the reaction of Rh and CH2Cl2. The insertion complex is again not identified in the matrix infrared spectra parallel to the tetra- and tri-halomethane cases described above, while the methylidene and insertion products are virtually the same, 57.8 and 57.3 kcal/mol more stable than the reactants. It is also notable that unlike the small Rh tetra- and tri-halomethylidenes, the Rh di-halomethylidene has two almost isoergic conformations, the staggered allene-type and planar (56.4 kcal/mol lower than the reactants) structures (Figure 9). However, the planar CH2dRhCl2 molecule would have a strong CH2 wagging band near 1000 cm-1 and a reasonably strong C-Rh stretching band at 750 cm-1, both of which are not observed in this study. Evidently only the allene-type CH2dRhCl2 structure is produced in a measurable amount.

Cho and Andrews

Figure 5 shows the product spectra from reactions of Rh with CH2FCl and its deuterated isotopomers, where both m and i absorptions are observed. The m absorptions increase ∼5% on visible irradiation but decrease ∼30% on UV photolysis and slightly recover on the following visible irradiation, similar to the CHCl3 and CH2Cl2 cases. The unique, strong m Rh-F stretching absorption of CH2d RhFCl is observed at 591.3 cm-1 (Table 5), which shows a small D shift of 0.5 cm-1. They are compared with B3LYP calculated frequency and D shift of 600.8 and 0.8 cm-1. A weak m absorption is observed at 928 cm-1, while the D counterpart expected at ∼730 cm-1 is apparently covered by precursor absorption. The good correlation with the predicted values and similar photochemical properties to the CHCl3 and CH2Cl2 products support generation and trapping of CH2dRhFCl. The i (i for insertion product) absorptions slightly increase on visible irradiation, increase ∼15% on UV photolysis, and remain almost the same on the following visible irradiation. The strong i absorption observed at 967.7 cm-1 (with a shoulder at 970.5 cm-1) has the D counterpart at 940.8 cm-1 (with a shoulder at 942.6 cm-1) (H/D ratio of 1.029), and we designate it as the C-F stretching mode of CH2F-RhCl on the basis of the frequency and small D shift, while the observed frequency is slightly higher than the B3LYP values (Table 6). The i absorption at 1217.9 cm-1 shows a large D shift of 263.6 cm-1 (H/D ratio of 1.276) and is assigned to the CH2 wagging mode. Identification of the insertion products in the CH2FCl spectra is consistent with the relative stability of the methylidene and insertion products, which are 47 and 42 kcal/mol more stable than the reactants. The CH2dRhFCl methylidene is also computed to have an allene-type structure, parallel to the Rh methylidenes described above. Planar CH2dRhFCl, which is 36 kcal/ mol more stable than the reactants, is 10 and 5.4 kcal/mol higher in energy than the identified products, and the predicted strong CH2 wagging and Rh-F stretching bands at 950 and 550 cm-1 are not observed. The product spectra from reactions of Rh with CH2F2 and its deuterated isotopomers are shown in Figure 6, where only m absorptions are observed. They remain almost the same on visible irradiation but triple on the following UV photolysis and slightly decrease on full arc irradiation. The most distinctive m absorption is observed at 634.9 cm-1 (with a shoulder at 637.9 cm-1) with D counterpart at 632.8 cm-1 (with a shoulder at 635.8 cm-1) (H/D ratio of 1.003). It is assigned to the RhF2 antisymmetric stretching mode of the staggered allene-type CH2dRhF2 molecule. The frequency and D shift of 2.1 cm-1 are well reproduced by calculation (e.g., B3LYP values of 647.0 and 1.8 cm-1). The m absorption at 909.6 cm-1 (with a shoulder at 911.5 cm-1) is assigned to the CH2 wagging mode, while the D counterpart expected at ∼740 cm-1 is covered by precursor absorption. A weak m absorption at 734.4 cm-1 is designated as the CH2 rocking mode without observation of the D counterpart, and the band at 567.8 cm-1 in the CD2F2 spectra is assigned to the RhF2 symmetric stretching mode without observation of the H counterpart. The CH2dRhF2 and CH3F-RhF products are in fact energetically comparable, 24.6 and 25.1 kcal/mol more stable than the reactants, while the higher-oxidation-state complex is expected to be stabilized more in the matrix because of the more polarized molecular bonds. However, no i absorptions are observed in the CH2F2 spectra. Planar

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CH2dRhF2 (24.0 kcal/mol lower than the reactants) is again almost isoergic with the allene-type isomer; however, it would show the strongest RhF2 antisymmetric stretching mode near 610 cm-1 with a D isotopic frequency increase of 24 cm-1, which are not observed in this study. An interesting, strong absorption is observed at 611.1 cm-1 with no D shift, next to the RhF2 antisymmetric stretching band of CH2dRhF2. It appears only in the CH2F2 and CD2F2 spectra, and it remains the same on visible irradiation but increases 70% on subsequent UV irradiation. We tentatively assign this band to the antisymmetric stretching mode of isolated RhF2 on the basis of the B3LYP calculated value of 641 cm-1. Such RhF2 absorption is more evidence for formation of the small Rh methylidene, CH2dRhF2. In a recent study, NiCl2 absorptions are observed in the product spectra from reactions of Ni with CH2Cl2 isotopomers, while the Ni methylidene is not identified, indicating that CH2-NiCl2* is generated, but is, however, not stable enough to be captured in the matrix and dissociates to form NiCl2.8a In contrast, CH2dRhF2* dissociates only in part to produce RhF2 during relaxation in the matrix. Rh þ CH3F. Figure 7 shows product spectra from the reaction of Rh with CH3F and its deuterated isotopomers, where only i primary product absorptions are observed in contrast to the tetra-, tri-, and di-halomethane cases. The i absorptions double on visible irradiation and increase another 100% on UV irradiation (∼300% increase in total). The relatively strong i absorption at 1183.1 cm-1 is partially overlapped with the CHF precursor photolysis product26 absorption at 1181.5 cm-1 and has the D counterpart at 920.5 cm-1 (H/D ratio of 1.285). It is assigned to the CH3 deformation mode of the CH3-RhF insertion product due to the frequency and large D shift. The distinctively strong i absorption at 559.8 cm-1 (with a site absorption at 550.5 cm-1) has its D counterpart at 552.6 cm-1 (with a site absorption at 546.1 cm-1) and is assigned to the Rh-F stretching mode on the basis of the frequency and small D shift. Another i absorption is observed at 2199.5 cm-1 in the CD3F spectra and is designated to the CD3 antisymmetric stretching mode. However, its H counterpart, which is expected at 2900 cm-1, is probably covered by precursor absorption in this crowded region. The strong, observed i absorptions are consistent with the stability of the insertion product over the methylidene complex: CH3-RhF and CH2dRhHF are 28 and 17 kcal/mol more stable than the reactants. However, it is interesting that the RhH and RhD diatomic molecule absorptions25 are observed at 1920.6 and 1379.9 cm-1, as shown in Figure 7, while no other rhodium hydrides are identified. A previous Rh þ H2 study clearly shows that the major products are RhH2 and (H2)RhH2, and the RhH absorption is relatively weak.25 The weak RhH and RhD absorptions observed in the CH3F and CD3F spectra suggest that a primary species with a Rh-H bond is also formed during the reaction of Rh with CH3F. The higher energy methylidene CH2dRhHF is unlikely, and even less likely to decompose to the less stable RhH than the more stable RhF diatomic molecule. In the Pt case, both CH3-PtF and CH2F-PtH were observed.8d It is therefore likely that some CH2F-RhH (20 kcal/mol more stable than reactants) is also formed in addition to the more stable CH3-RhF isomer, and this unstable species even(25) Wang, X.; Andrews, L. J. Phys. Chem. A 2002, 106, 3706, and references therein (Rh þ H2).


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tually dissociates into RhH and CH2F. The CH2F absorption is also observed at 1162.2 cm-1.26 Finally, weak trace RhNN and RhCO bands were detected.27,28 Rh þ C2H6. Figure 8 shows the product spectra from reactions of Rh with ethane and its deuterated isotopomers, where only i absorptions are observed, parallel to the CH3F case. The i absorptions remain unchanged on visible irradiation, increase ∼5% on UV irradiation, and slightly increase on full arc photolysis. A group of i absorptions in the range 2030-2053 cm-1 have their D counterparts in the region 1461-1475 cm-1 (H/D ratio of 1.389-1.392), suggesting various conformations or trapping sites for the product in the matrix. Their intensities substantially rearrange in the process of gradual annealing, leading to the strongest absorption at 2030.9 cm-1 and its D counterpart at 1461.5 cm-1 (H/D ratio of 1.390). These observed frequencies are compared with the recently reported Rh-H stretching frequency of 2035.1 cm-1 for CH3-RhH and its D counterpart of 1464.3 cm-1, which also emerge in the process of annealing.11 These new absorptions are also compared with the previously observed absorptions at 2099.4 and 2053.4 cm-1 for RhH2, 2014.0 cm-1 for (H2)RhH, 1980 cm-1 for (H2)RhH2, 1512.2 and 1475.2 cm-1 for RhD2, 1423.7 cm-1 for DRh(D2), and 1422 cm-1 for (D2)RhD2.25 These i absorptions in the Rh-H stretching region indicate that a primary product with a Rh-H bond is formed, whose frequency is close to the Rh-H stretching frequency of CH3-RhH.11a Moreover, the observed frequencies are 0.957-0.960% of the B3LYP frequency of 2122.1 cm-1 for C2H5-RhH, which is in excellent agreement. Accordingly the Rh-H stretching absorptions are attributed to the insertion product formed from reaction of Rh and C2H6. Another group of i absorptions are observed in the range 1160-1169 cm-1 and their D counterparts in the region 946-953 cm-1. Subsequent annealing leads to the strongest band at 1160.0 cm-1 and its D counterpart at 946.1 cm-1 (H/D ratio of 1.226). They are designated to the CH2 wagging mode on the basis of the frequencies and isotopic ratio (e.g., B3LYP frequency of 1206.3 cm-1 and H/D ratio of 1.217). The Rh-H stretching and CH2 wagging bands are in fact the strongest ones for the insertion complex, as shown in Table 9, while the observably strong C-H stretching absorptions are probably covered by precursor absorption in the crowded area, and the CRhH bending band is too close to our observation limit. Identification of C2H5-RhH is also consistent with its low energy relative to the methylidene complex: C2H5-RhH is 14.2 kcal/mol more stable than the reactants, whereas CH3CHdRhH2 is 2.6 kcal/mol higher. The cyclic dihydrido complex (CH2CH2-RhH2), another possible product, is 17.4 kcal/mol more stable than the reactants. However, its strongest C-H stretching band, expected at ∼2860 cm-1, is not observed. It is also interesting that RhH and RhD absorptions are observed at 1920.6 and 1379.9 cm-1 without other rhodium hydride absorptions, parallel to the CH3F case, and they increase ∼10% during photolysis. The appearance of RhH in the ethane reaction product spectrum indicates that the rhodium insertion complex with excess energy (C2H5-RhH*) produced from the reaction of Rh and (26) (a) Jacox, M. E. Chem. Phys. 1981, 59, 199. (b) Jacox, M. E.; Milligan, D. E. J. Chem. Phys. 1969, 50, 3252. (c) Raymond, J. I.; Andrews, L. J. Phys. Chem. 1971, 75, 3235 (CH2F). (27) Wang, X.; Andrews, L. J. Phys. Chem. A 2002, 106, 2457 (Rh þ N2). (28) (a) Zhou, M.; Andrews, L. J. Am. Chem. Soc. 1999, 121, 9171. (b) Zhou, M.; Andrews, L. J. Phys. Chem. A 1999, 103, 7773 (RhCO).

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C2H6 and photolysis afterward dissociates in part into RhH and C2H5, which is also observed in the spectrum.8d The RhH diatomic molecule and CH3 radical absorptions along with CH3-RhH were observed on sample co-deposition in our analogous experiment with Rh and methane.11b Structures of Rh Complexes. The structures of the Rh complexes identified in this study are illustrated in Figure 9, where the products are grouped by reagent. The carbene complexes, CCl2dRhCl2, CFCldRhCl2, CF2dRhCl2, CHCld RhCl2, CH2dRhCl2, CH2dRhFCl, and CH2dRhF2, are computed to have staggered allene-type structures, parallel to the corresponding group 3, 8, and 10 metals.5,7,8 As described above, tetra-, tri-, and di-halomethanes form the small methylidene except for CH2FCl, revealing that the group 9 methylidenes are more favored than the group 10 methylidenes, particularly the Ni and Pd carbenes. Only the insertion products are identified from reactions of CH3F and C2H6, while both insertion and methylidene complexes are observed in the spectra of CH2FCl. The identified carbyne complexes, ClCtRhCl3 and FCtRhCl3, have Cs structures; one of the three Rh-Cl bonds differs from the other two, which are similar to those of the previously studied tetrahalo Ru and Os carbynes. Clearly the Rh-C bonds of the small Rh carbenes and carbynes are double and triple bonds; the bond lengths of 1.788-1.820 and 1.730-1.740 A˚ are compared with 1.988 and 2.007 A˚ for single Rh-C bonds in CH3-RhF and C2H5RhH. The bond lengths may also be compared with CdRh and C-Rh bond lengths of ∼1.91 and 2.0-2.2 A˚ measured for typical Rh complexes.29 We have found no evidence for ligated Rh carbyne compexes. Reactions. Recent investigations in our laboratory have shown that laser-ablated transition-metal atoms react with small alkanes and halomethanes through C-H(X) bond activation/insertion followed by R-H(X) transfer,4-9 reaction 1.

M þ CX4 f CX3 -MX f CX2 -MX2 f CX-MX3 ð1Þ The present results reveal that the small high-oxidation-state complexes are generated from reactions of Rh with halomethanes, and therefore, formation of small carbene or carbyne complexes in reaction with small alkanes and halomethanes is a general phenomenon for group 3-10 transition metals and actinides.4-9 The Rh reaction products are comparable with those in the Ru and group 10 metal cases. The preference for higher-oxidation-state products clearly decreases on moving right in the row. Ru generates small methylidynes as well as methylidenes in reactions of halomethanes,7 whereas Pd forms its methylidenes only in reactions of tetra-halomethanes.8b The observation of RhF2 in the CH2F2 experiments suggests that CH2-RhF2* formed in the primary reaction partly dissociates into the rhodium difluoride. Similarly the RhH absorptions observed in the CH3F and C2H6 spectra as described above show that products with a Rh-H bond are generated during the Rh reactions. In CH3F reactions, C-F bond insertion is believed to occur primarily because electron-rich F attracts the metal atom, forming the insertion complex (CH3-RhF) first, whose strong absorptions are observed in the spectra. Identification of RhH suggests that CH2F-RhH is also produced and eventually dissociates into RhH and CH2F, both of which are observed in the spectra. (29) (a) Kobayashi, J.; Nakafuji, S.-Y.; Yatabe, A.; Kawashima, T. Chem. Commun. 2008, 6233. (b) Yeston, J. S.; Bergman, R. G. Organometallics 2000, 19, 2947. (c) Mata, J. A.; Chianese, A. R.; Miecznikowski, J. R.; Poyatos, M.; Peris, E.; Faller, J. W.; Crabtree, R. H. Organometallics 2004, 23, 1253.

Cho and Andrews

On the other hand, RhH (and RhD) observed in the ethane spectra is indicative of formation the insertion complex, C2H5-RhH, because dissociation of the methylidene (CH3CHdRhH2) or cyclic dihydrido (CH2CH2-RhH2) complexes would produce RhH2 or H2 elimination from the dihydrido complexes and lead to formation of other rhodium hydrides. The rhodium dihydride absorptions expected at 2099.4 and 2053.4 cm-1 and the absorptions of the dihydrido products themselves are not observed in this study. Evidently part of the insertion complex dissociates due to excess internal energy from the initial reaction.

Conclusions Laser-ablated Rh atoms react with halomethanes and ethane, and the products are identified on the basis of frequencies, isotopic shifts, and correlation with the DFT results. The small Rh methylidenes and methylidynes are identified in the product spectra form reactions of tetra-, tri-, and di-halomethanes, and the calculated bond lengths of 1.788-1.820 and 1.730-1.740 A˚ are appropriate for Rh-C double and triple bonds, respectively. On the other hand, CH3-RhF is the primary product from the CH3F reaction. Likewise, the insertion complex C2H5-RhH is identified from reaction of ethane, whose observed Rh-H stretching frequency of 2030.9 m-1 (with D counterpart of 1461.5 cm-1) is compared with the previously observed Rh-H stretching frequency of 2035.1 cm-1 for CH3-RhH (with D counterpart of 1464.3 cm-1).11 Observation of the RhH diatomic without RhH2 provides evidence that the insertion complex (C2H5-RhH) does not proceed to the methylidene or cyclic dihydrido product (CH3CHdRhH2 or CH2CH2-RhH2) but dissociates in part into RhH and C2H5. The present results reveal that group 3-10 transition metals and actinides all undergo C-H(X) insertion and rearrangement to the high-oxidation-state complexes. However, the higher-oxidation-state complexes become less favored on going far right in the row. In the reactions with Rh, halomethanes form small methylidenes and methylidynes, but small alkanes generate only insertion complexes. The small Rh methylidenes are computed to have staggered allene-type structures, which are more stable than the planar structures, while the Rh methylidynes have Cs structures similar to those of Ru and Os methylidynes. In case of the di-halomethylidenes, the two structures are energetically close, but the observed frequencies correlate better with the allene-type structures. In the case of CCl2dRhCl2, the planar structure is 6 kcal/mol higher in energy, and the computed frequencies do not fit as well as those computed for the staggered structure. The C-Rh bonds of the identified methylidenes and methylidynes are substantially shorter than those of the insertion complexes, suggesting that the C-Rh bonds indeed have double- and triple-bond character.

Acknowledgment. We gratefully acknowledge financial support from National Science Foundation (U.S.) Grant CHE 03-52487 to L.A. and support from a Korea Research Foundation (KRF) grant funded by the Korean government (MEST) (No 2009-0075428). Supporting Information Available: Tables S1-S9 of calculated frequencies for unobserved intermediate species. This material is available free of charge via the Internet at http:// pubs.acs.org.

Infrared Spectra of Simple Methylidyne, Methylidene, and Insertion

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